Microelectronic devices are typically provided in units or packages that provide physical and chemical protection from the environment. Such a package typically includes a thin dielectric substrate having electrically conductive terminals thereon. A microelectronic element such as a semiconductor chip, which is typically also thin, is mounted upon the substrate and is electrically connected with the terminals on the substrate. An encapsulant commonly covers at least a part of the microelectronic element, and may also cover features such as wire bonds which form part of the connection between the semiconductor chip and the terminals of the substrate. Such packages can be stacked one atop another in a common assembly, resulting in significant space saving on a circuit board. The surface area of such a stacked assembly is equal to or only slightly greater than the surface area of an individual unit.
One type of stacked package assembly known heretofore is sometimes referred to as a “ball stack.” A ball stack assembly includes two or more individual units. Each unit includes a substrate and one or more microelectronic elements, which are mounted upon the substrate and electrically connected to the terminals on the substrate. The individual units are stacked one atop another, and the terminals of each individual unit are electrically coupled with terminals of other individual units by electrically conductive bonding material such as solder balls, thus forming a plurality of vertical conductors. The terminals of the bottom unit may constitute the terminals of the entire assembly, or, alternatively, an additional substrate may be mounted at the bottom of the assembly which may have terminals connected to the terminals of the various unit substrates. Ball stack packages are depicted, for example, in certain preferred embodiments of U.S. Published Patent Application Nos. 2003/0107118 and 2004/0031972, the disclosures of which are hereby incorporated by reference herein. The individual units in stacked unit assemblies can be permanently bonded in place by heating the assembly so as to melt or to reflow the solder balls, or to otherwise activate the bonding material.
However, an effective bonding process requires proper alignment of the individual units relative to each other. Misalignment of individual units can lead to improper solder connections between the units, which can result in electrical shorts or disconnects. Accordingly, there is room for improvement in stacking and aligning individual units such that the reflow process results in effective and reliable conductive bonds between the terminals of the individual units.